Gas spectrometers are utilized in a wide variety of industrial and medical applications to monitor the presence and concentration of one or more predefined components in a gas sample. Typically, light of a known spectral content is directed through a gas sample and the intensity of the transmitted light at a number of different center-wavelengths is detected. By utilizing known light absorption characteristics of the predefined gas components at the center-wavelengths, the detected light intensities provide a basis to determine, via statistical processing, the concentrations of the predefined components. As will be appreciated, it is important that the initial calibration conditions of the spectrometer be maintained in order to accurately relate the measured light intensities to gas component concentrations.
This is particularly true in respiratory gas spectrometers for measuring the concentration of carbon dioxide and/or oxygen, and one or more anesthetic agents such as nitrous oxide, halothane, enflurane, isoflurane, sevoflurane and desflurane in a respiratory gas stream. In such applications, a separate sample stream is typically drawn from the patient respiratory gas assembly and directed into a sample chamber that is positioned on the optical path between the light source and detector.
It is particularly important in respiratory gas spectrometry that any significant absorbers of light at the center-wavelengths of interest that are on the optical pathway between the light source and detector be accounted for in calibration, and that the related calibration conditions be maintained during use. In this regard, the optical pathway(s) utilized in many respiratory gas spectrometers lie substantially within a sealed sample gas chamber. Further, given the responsivity needs of respiratory gas spectrometers, it is also important that the transmitted light reaching the detector be of an intensity that yields an acceptable signal to noise ratio.